downsizing mpeg bitrate

Hey guys; I have mpeg2 files of feature films for a dual layer dvd product, the files were used for digital cinema screenings so they are encoded at around 29 mb/ps (weighing in at around 20 -25 gb). I need to re-encode these files to fit on dual layer DVDs; I'm working on DVD studio pro which won't allow bitrate changes to files that are already encoded. I've tried using compressor but the resulting files will encode to a certain point and then just crash or just shows a black screen with no audio. I've gotten around this in the past by converting the files into other formats and then re-encoding but I'm sure there's a more efficient method; I've searched around but found no information...anybody ?
Cheers
-Rob

I'm working on a mac 2.8ghz core 2 duo, 4gb ram
with final cut studio products.